The present invention relates to method and apparatus for compensating for Polarization Mode Dispersion (PMD) in high speed optical communication systems.
Polarization Mode Dispersion (PMD) compensation is becoming an urgent issue because it will be the most important limiting factor for high speed optical transmission systems (e.g., OC192 or OC768 systems). For standard single mode optical fibers, the transmission distance of, for example, an OC192 system is limited to 400-600 kilometers due to pulse distortion caused by PMD. Therefore, PMD compensation is very important in upgrading existing system capacity to or beyond the OC192 system level.
Referring now to FIG. 1, there is shown an exemplary optical signal that is affected by PMD before PMD compensation is applied, and the same signal after PMD compensation. More particularly, PMD causes an optical pulse to be split into two orthogonally polarized pulses with a differential group delay between the two pulses as the optical pulse propagates down an optical fiber. This is shown by the two pulses indicated as xe2x80x9cBefore Compensationxe2x80x9d in FIG. 1. Since receivers in most optical transmission systems are polarization independent, a detected signal will be distorted due to the differential group delay. PMD compensation is a technique which returns the two polarized pulses back into a single in-phase pulse, as is shown by the single pulse indicated as xe2x80x9cAfter Compensationxe2x80x9d in FIG. 1, before further transmission or processing in the high speed optical communication system.
Current compensation schemes require a direct measurement of the PMD value, which is very complicated and slow. Since all compensation schemes are based on the assumption of the existence of principal states of polarization (PSP""s), the implementation of PMD compensation requires endless polarization tracking of the PSP""s. These compensation schemes are not compatible with other polarization control techniques that use scrambling since the data rate of the scrambling techniques are much higher than the response times of current PMD compensators. Other disadvantages of the current PMD compensators include, for example, complicated optical design, mechanically moving parts, high insertion loss, and high cost.
It is desirable to provide a Polarization Mode Dispersion (PMD) compensator for use in high speed optical transmission systems, where the compensator has a simple optical design, a faster response time, and a low cost as compared with known PMD compensation arrangements.
The present invention is directed to method and apparatus for compensating for Polarization Mode Dispersion (PMD) in high speed optical communication systems. More particularly, the present invention relates to Polarization Mode Dispersion (PMD) compensators using an interferometer arrangement for use in high speed optical communication systems.
Viewed from one aspect, the present invention is directed to a polarization mode dispersion (PMD) compensator for compensating for PMD occurring in an optical input transmission line. The PMD compensator comprises optical circuitry, and an interferometer arrangement. The optical circuitry receives from the optical input transmission line an input signal comprising first and second PMD generated associated optical pulses having first and second principal states of polarization, respectively, and directs the received first and second PMD optical pulses with their first and second principal states of polarization, respectively, onto respective first and second paths. The interferometer arrangement comprises first and second paths that are coupled to the first and second paths, respectively, of the optical circuitry for propagating the respective first and second PMD optical pulses. The interferometer arrangement measures a PMD differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensates for the measured differential delay.
Viewed from another aspect, the present invention is directed to a polarization mode dispersion (PMD) compensator for compensating for PMD occurring in an optical input transmission line. The PMD compensator comprising optical circuitry, an interferometer arrangement, an optical combiner, and a control arrangement. The optical circuitry receives from the optical input transmission line an input signal comprising first and second PMD generated optical pulses having arbitrary orientations of first and second principal states of polarization, respectively. The optical circuitry selectively adjusts the aligning of the received arbitrarily oriented first and second principal states of polarization to predetermined fixed first and second principal states of polarization required for directing the first and second PMD optical pulses onto respective first and second output paths. The interferometer arrangement comprises first and second paths that are coupled to the first and second output paths, respectively, of the optical circuitry for propagating the respective first and second PMD optical pulses. The interferometer arrangement measures a PMD differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensates for the measured differential delay. The optical combiner optically combines the signals from the first and second paths of the interferometer arrangement to generate a combined output signal. The control arrangement selectively controls the compensation for a measured differential delay between the first and second polarization states in the interferometer arrangement in response to at least a portion of the combined output signal from the optical combiner, and for selectively adjusting the aligning of the received first and second polarizations to the required predetermined fixed first and second polarizations in the optical circuitry.
Viewed from still another aspect, the present invention is directed to a method of compensating for polarization mode dispersion (PMD) produced in a transmission line. In a first step of the method, an input signal is received comprising first and second PMD generated optical pulses having arbitrary orientations of their first and second principal states of polarization, respectively. In a second step of the method, the arbitrarily oriented first and second principal states of polarization are adjusted in a polarization controller to match orientations of predetermined fixed first and second principal states of polarization of a polarization beam splitter. In a third step of the method, the first and second PMD optical pulses with the adjusted first and second principal states of polarization, respectively, are directed onto respective first and second output paths of the beam splitter. In a fourth step of the method, a PMD differential delay between the predetermined fixed first and second principal states of polarization of the first and second PMD optical pulses, respectively, is measured. In a fifth step of the method, the measured differential delay from the fourth step is selectively compensated for in an interferometer arrangement comprising first and second paths that are coupled to the first and second output paths, respectively, of the polarization beam splitter.
The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.